SETI, the search for extraterrestrial intelligence, has been conducted on the earth for about half a century. It started in 1960 with Project Ozma, an attempt by radio astronomer, Frank Drake, to listen for intelligent radio signals from two nearby stars, Tau Ceti and Epsilon Eridani.

The time-varying signal intensity, 6EQUJ5, was recorded using a 36 level coding with the characters (space), 1..9, A..Z.(Via Wikimedia Commons))

Such radio searches are one-way affairs; that is, we're just listening and hoping that the extraterrestrials are kind enough to send some very strong radio signals our way. Perhaps the extraterrestrials are not in a talking mood. Aside from the asymmetry of this scheme, there's the problem that we think that radio is the best means for such extraterrestrial contact.

I'm reminded of the golden hammer rule; namely, "When all you have is a hammer, everything looks like a nail." Will radio even be used a hundred years from now, or will it have been replaced by something much better? A hundred years from now our descendants might be comparing their technology to radio in the same way that we compare radio to smoke signals.

Mathews thinks that such probes should start as dual-purposed objects that would be designed to clear debris from our solar system. I mentioned the dangers that Earth faces from rogue asteroids in a previous article (Asteroid Deflection, April 19, 2012). Such autonomous, self-replicating robots can reproduce from materials found in our solar system to launch into the vicinity of other stars.[4]

A recent numerical simulation of exploration by such SETI robots gives some interesting insights into the probability of making contact.[5] The model assumes that a probe will have a lifetime of 50 million years, and there's a contact window of about a million years. If there were between a hundred and a thousand civilizations doing robotic probing, then we would have heard from one of these if they were within a thousand parsecs of the Earth. The extent of the Milky Way Galaxy is about 35,000 parsecs.